Natural Ester Oils or Esters are used in cooling Transformers. These ester based oils are created by a reaction between a tri-alcohol and fatty acids. Natural Oils have higher flash and fire point. They are chemically stable. They are also biodegradable.
Another important advantage is the water solubility. Natural ester oils have a water solubility which is 20 to 30 times that of mineral oils. Thus water is drawn from the insulation and absorbed in the oil. This increases the life of the insulation. They are thus considered an effective alternative to mineral oils which are environmentally risky and prone to fire accidents.
The downside is that Natural Ester Oils are more expensive. They also have high viscosity which results in slower flow through the transformer and results in reduced heat transfer. Natural Ester Oils are also prone to oxidation which reduces the lifespan. Natural ester oils are used in temperate climates. At cold climates, the flow rate is reduced.
Synthetic Ester based oils are artificial oils made by select ingredients. These oils can be tailored to meet the exact requirements of the application. Synthetic ester oils can be made to resist oxidation. They can be used in cold as well as temperate climates.